Overtraining is very real. In 2006 I decided I wanted to get bigger and hit 185 lbs, I was 165 at the time. Coupled with a solid diet, I started with intelligently designed workouts and was in the gym 4 times weekly, 1-2 hrs per session. After 6 weeks I felt I was getting mediocre results so I figured I would train more. I upped my calories and protein intake and started training 5 times a week, then after two months I was only taking one rest day a week. No matter how much I varied my workouts I was still feeling off, my gains came to a hault and I basically I fried my CNS. Reluctantly, I took a week off and then trained hard 3 times a week. After 3 moths I exploded, gained double what I gained in the 5 months prior and was hovering at close to 190 lbs not to mention got much stronger. Based on that experience I now train 3-4 times a week and take 1-2 weeks off every 8-12 weeks. This i truly believe has allowed me to get big and strong. So this is what's right for me and it's a very individual thing. I know guys (not on gear) who can do 5 days a week long term with no problems.